Vibrational spectroscopic techniques such as Raman Spectroscopy in its various forms, including resonance Raman and surface enhanced Raman spectroscopy (SERS), are well-known for identifying samples, which contain unknown materials or substances, ranging from simple molecular compounds to more complex structures. Since different samples exhibit unique vibrational spectra, harmful materials, for example, can be identified by detection devices, such as Raman or IR devices, without the need to physically contact the material or add labeling chemical agents. This feature permits law enforcement personnel, public health agencies, medical personnel, and the military to identify contraband narcotics, explosives, poisons, pathogens, and toxic chemicals with minimal risk. In SERS, as a molecule approaches a roughened metal surface, the intensity of its Raman spectra is enhanced. Thus, SERS offers the advantage of sensitivity in addition to the specificity of normal Raman or IR.
However, conventional spectral signal analysis techniques often cannot distinguish spectra between closely-related samples. Thus, identification errors can occur when spectroscopic signatures are produced for closely-related spectra. For example, two different spectra may incorrectly appear to be identical on a principle component analysis (PCA) plot.